1. Field Of The Invention
This invention relates to semiconductor devices and, more particularly, to the simultaneous formation of bipolar and metal oxide semiconductor (MOS) devices on the same silicon substrate.
2. Description Of The Relevant Art
Methods for the fabrication of bipolar devices and MOS devices are known, Typically, bipolar devices are fabricated separately from MOS devices because of the structural differences between the two types of devices. This means that a circuit which uses both bipolar and MOS devices must be constructed using discrete chips, which increases the size and cost of the product.
If bipolar and MOS devices are to be combined, an integrated approach to device fabrication must be devised. However, applying the fabrication techniques used for one type of device to another type of device typically degrades the performance of the other device. For example, a common method for making electrical contact to a silicon substrate in a bipolar transistor involves the use of a layer of polysilicon deposited on the surface of the substrate. Electrical contact to the silicon substrate is made through this layer of polysilicon. The resulting structure is termed a "buried contact." However, the polysilicon/silicon interface layer increases the series resistance through the device. This is insignificant in bipolar devices, because the base of bipolar devices has small current flowing through it, and bipolar devices have intrinsically high resistances. However, the source and drain in MOS devices carry all of the current, so any increase in series resistance significantly affects performance of the devices. Series resistance can be improved by increasing the contact area, but with a resulting decrease in yield. Finally, the thin gate oxide layer used in MOS devices is subject to contamination and mechanical damage when subjected to bipolar fabrication methods.